This invention relates to the field of jet deflection and has specific application to a recording or printing process wherein one or more jets of recording liquid are controlled to reproduce graphic information on a print receiving medium. An early example of such a device is disclosed in Ranger et al U.S. Pat. No. 1,817,098, wherein a pair of electrodes are positioned on opposite sides of a jet. Control signals are applied to the electrodes to deflect the jet against a catching surface for achieving a no-print condition. A print receiving medium is transported through the active printing region of the jet (or three such jets of different colors), and pictorial information is reproduced by programmed application of no-print or catching signals to the control electrode. Hansell U.S. Pat. No. 1,941,001 discloses a somewhat more advanced symmetrical electrode arrangement for use in facsimile recording.
The Ranger and Hansell patents relate to the control of jets comprising continuously flowing streams, which break up into drops on a random basis in accordance with physical principles described by Lord Rayleigh in the middle of the 19th century. More recently the quality of jet drop recording has been improved by stimulating the stream at a frequency near its natural frequency to create drops which are uniformly sized and regularly spaced. An application of such a technique to character printing is disclosed in Lewis et al U.S. Pat. No. 3,298,030.
In the Lewis patent a plurality of streams are stimulated for drop generation, and the drops are directed toward laterally separated positions by charging the drops to different levels and directing them all through electrostatic deflection fields. Under the action of the deflection fields all drops are deflected in proportion to their level of charge. A series of charge rings are utilized for drop charging and the deflection fields are produced by a pair of deflection electrodes. Character generating circuitry causes the printing of predetermined characters by selectively applying a catching potential to the charge rings. Drops which are charged in response to this catching potential are deflected into an appropriately positioned catcher.
A somewhat more compact arrangement for jet drop printing is disclosed in Sweet et al U.S. Pat. No. 3,373,437, wherein a row of closely spaced streams are produced by a common manifold and are commonly stimulated for drop generation. A series of charging electrodes charge the drops on a binary basis in response to desired "print" or "no-print" conditions. The "no-print" drops are charged to a predetermined level for deflection into a catcher. The "print" drops are uncharged.
The recording head arrangement taught by Sweet et al has been improved and made more compact by using a laminated plate arrangement, including a notched charge plate as shown in Culp U.S. Pat. No. 3,618,858. For application of the Sweet et al technique to a twin row print head, reference may be made to Mathis U.S. Pat. No. 3,701,998. Utilization of the Mathis invention requires a row-to-row switching delay as taught in Taylor et al U.S. Pat. No. RE28,219.
It will be appreciated that binary or on/off printing as taught by Sweet et al requires extremely close jet spacing for high resolution printing. As taught in King U.S. Pat. No. 3,739,395, such spacing requirements may be somewhat relaxed by lateral drop deflection in a manner similar to that of Lewis et al. As shown in King, selective drop catching is achieved by deflection in a direction perpendicular to the lateral scanning direction.
Another patent, Robertson U.S. Pat. No. 3,656,171, teaches a recording technique which eliminates the requirement for drop deflection fields, thereby enabling closer spacing than electrical considerations previously would permit. The Robertson technique, however, is an on/off process, so there remains a packing density problem caused by the necessity of providing apparatus for creating a very great number of closely spaced jets.